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I have read and heard about this problem for years but this year is the first time I have encountered it, personally. The runner beans are simply dropping all their flowers leaving funny looking stalks on the branches where there should be loads of beans. I water them every evening and there are bees busying themselves on the flowers. I feed the ground regularly, the weather has been hot, but not too excessive. There do not appear to be any particular signs of predators either. I am at a loss…

The explanation seems to be that this just happens sometimes! It is still early in the season, however and the best help I have found is this excellent summary and advice page from the Royal Horticultural Society…

What is the problem?

Runner beans can often fail to set pods due to a number of reasons.
Failure of runner beans to set pods is a surprisingly widespread problem, particularly early in the season. Attention to soil conditions can help increase the harvest.

Even under ideal conditions, sometimes more than half of runner bean flowers may fail to set bean pods. If significantly more than this fail to set pods, yield is noticeably reduced.

Controlling the problem

The following steps have been shown to help improve pod set in runner beans:

Plant or sow runner beans into soil that has had plenty of organic matter, such as well-rotted manure, added the previous autumn, as this will aid moisture and nutrient retention around the roots
Lime the soil before planting if the pH is below 6.5
Apply a mulch of organic matter over the root area, to help conserve moisture
Plant in a sheltered site, as this will encourage bees to visit and pollinate the plants
Rotate the crop each year, never growing it in the same site for two years running. This will help avoid build-up of pest and disease problems
Water in dry weather, giving 5-11 litres per sq m (1.1-2.2 gallons per 10 sq ft) twice a week
Don’t mist or syringe the flowers with water (as used to be advocated) as this has not been shown to help pod-set
Take care when applying treatments for aphids or other insect pests of runner beans, because the pesticide could also kill bees and other pollinating insects. Spraying at dusk will reduce danger to bees
Where birds are a problem, try growing a variety with different colour flowers the following year, as this may deter the bird pests. If this does not work, consider growing a dwarf runner bean variety such as ‘Hestia’ or ‘Pickwick’, which can be netted against birds

What causes the problem?

The main causes of failure to set pods are:
Lack of moisture at the roots
Poor soil or growing conditions, such as acid soils below pH 6.5, pest or disease problems, frost damage, lack of nutrients or organic matter
Lack of pollinating insects, perhaps because of cold, wet or windy weather
Very hot weather, especially at night, which inhibits the germination of pollen grains, interrupting the pollination and fruit-set process. Cropping should resume in September, once the nights start to cool down
The presence of more attractive flowers for pollinating insects nearby. The number of pollinating bees visiting runner bean flowers often increases after the end of July, when more attractive sources of nectar such as white clover and lime trees have finished flowering
Nectar-robbing bees, which rob the sweet nectar from the runner bean flowers by biting a hole at their base, without entering the mouth of the flower and coming into contact with the pollen-producing parts. Nectar-robbing makes flowers unattractive to pollinating bees and insects
Birds, especially house sparrows, which peck at the flowers, making them harder for insects to pollinate
Infrequent harvesting: it is important to pick beans every two or three days, otherwise the pods will start to swell and set seed, which inhibits further pod production